Nonaqueous-electrolyte batteries including lithium secondary batteries are being put to practical use in extensive applications ranging from power supplies for so-called domestic use, e.g., ones for portable telephones and notebook type personal computers, to on-vehicle power supplies for driving, e.g., ones for motor vehicles. However, the recent desire for higher performances in nonaqueous-electrolyte batteries is growing more and more, and improvements in battery characteristics are required.
The electrolytes for use in nonaqueous-electrolyte batteries are usually constituted mainly of an electrolyte and a nonaqueous solvent. As main components of the nonaqueous solvent, use is being made of compounds such as cyclic carbonates, e.g., ethylene carbonate and propylene carbonate; chain carbonates, e.g., dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; and cyclic carboxylic acid esters, e.g., γ-butyrolactone and γ-valerolactone.
Various investigations are being made on nonaqueous solvents and electrolytes in order to improve battery characteristics of such nonaqueous-electrolyte batteries, such as load characteristics, cycle characteristics, and storability.
Patent document 1 proposes the use of ethyl methyl carbonate and dimethyl carbonate in order to inhibit the deterioration in overcharge characteristics and the deterioration through standing in a high-temperature environment which are attributable to the reaction of diethyl carbonate with lithium.
Patent document 2 proposes the use of a mixture of an asymmetric chain carbonate and a cyclic carbonate having a double bond as a nonaqueous solvent. There is a statement thereinto the effect that the cyclic carbonate having a double bond reacts preferentially with the negative electrode to form a coating film of satisfactory quality on the surface of the negative electrode and this inhibits the asymmetric chain carbonate from forming a nonconductor coating film on the surface of the negative electrode, whereby storability and cycle characteristics are improved.
Patent document 3 proposes that an additive which polymerizes at a battery voltage not lower than a maximum operating voltage of a battery is incorporated into the electrolytic solution to thereby enable the battery to increase in internal resistance and thus protect the battery. Patent document 4 proposes that an additive which polymerizes at a battery voltage not lower than a maximum operating voltage of a battery to cause gas evolution and a pressure increase is incorporated into the electrolytic solution to thereby enable an internal circuit breaker disposed for protection against overcharge to work without fail. Disclosed as these additives are aromatic compounds such as biphenyl, thiophene, and furan.
Patent document 5 discloses that a lithium secondary battery having a high capacity and excellent cycle characteristics can be provided by using an electrolytic solution containing an alkylene carbonate having a fluorine group, such as cis-4,5-difluoro-1,3-dioxolan-2-one or trans-4,5-difluoro-1,3-dioxolan-2-one.    Patent Document 1: JP-A-7-14607    Patent Document 2: JP-A-21-185806    Patent Document 3: JP-A-9-106835    Patent Document 4: JP-A-9-171840    Patent Document 5: JP-A-2004-319317